1. Field of the Invention
This invention relates to electrodes, in particular gas diffusion electrodes, and especially to bifunctional gas diffusion electrodes, having an improved structure which permits them to operate with increased power generation over a prolonged life span.
2. Description of the Prior Art
Bifunctional gas diffusion electrodes for metal/air batteries generally consist of three components. These components are a hydrophobic layer which permits air passage while retaining electrolyte, a dual component active layer attached thereto containing a catalytic active paste material, and a plurality of porous fiber metal current collectors in which the active paste is contained, such as shown in Chottiner et al. in U.S. Pat. No. 4,152,489.
The active paste material usually has an oxygen absorption/reduction catalyzed carbon (C) having a total surface area from 30 to 1500 square meters per gram (m.sup.2 /gm), a suitable catalyst such as silver (Ag) for oxygen reduction and decomposition of intermediate reaction components, an oxygen evolution metal-containing additive such as tungsten sulfide (WS.sub.2) or tungsten carbide (WC) coated with 1 to 20 weight percent (wt.%) cobalt (Co), and a dispersion of polytetrafluoroethylene (PTFE) as a bonding/non-wetting material, as taught by Chottiner et al. in U.S. Pat. No. 4,152,489, and Buzzelli in U.S. Pat. No. 3,977,901. This mixture is blended with deionized water to form the active paste.
Due to the relatively high viscosity of the active paste, it has been necessary to apply considerable force during the pasting operation in order to get reasonable loading of the porous fiber metal screen with active paste. This force generally tends to compress and compact the screen structure to provide only about 40% to 65% porous screen, and prevents the paste from filling all of the original screen pores or voids.
The ideal incorporation of the catalytic paste into the screen would be to have about 95% to 100% of the screen pores filled. This would produce the highest number of active sites within the confines of the screen, with all active sites very close to the metal collecting fibers. It is also advantageous to have most of the active material within the screen, rather than forming a separate composite layer held or attached to the screen structure. Thick coatings of catalytic paste on the surface of the screen result in many of the active sites being far removed from the nickel fibers with electrons from these sites having to traverse a high resistance path to each current collector.
For good cell performance, the electrolyte must penetrate into the electrode sufficiently to reach the interior surfaces, and contact air or oxygen in the presence of a catalyst. The electrode must at the same time be sufficiently electrolyte-repellent to prevent electrolyte flooding of the electrode pores. Electrolyte flooding can be a problem with gas diffusion electrodes, and while the Chottiner et al. structure, and the Buzzelli active paste composition, solve the problem to an acceptable degree by providing stable electrical characteristics for about 100 cycles, more improved structures or compositions would be highly desirable, especially if electrolyte flooding could be completely eliminated. Another problem with gas diffusion electrodes is the progressive dissolution of the discharge/oxygen reduction catalyst, particularly Ag, into the electrode during charging.
Typically, such electrodes have third cycle charging potentials of about 550 mV to 610 mV compared to a Hg/HgO reference electrode. Values of about 550 mV to 585 mV have been achieved using major amounts of oxygen evolution material, such as tungsten carbide (WC), adding substantially to the cost and weight of the electrode. It is desirable to lower this charging voltage, to conserve energy, and to reduce the amount of silver catalyst that dissolves in the electrolyte at that voltage. It is also desirable to reduce the cost and weight of the above described electrodes while maintaining a proper balance of electrolyte permeability.
Darland, Jr. et al. in U.S. Pat. No. 3,423,247 attempted to solve electrolyte flooding of gas diffusion electrodes in fuel cells by providing two zones in the electrolyte structure. One zone, next to the air supply, consists of low-surface-area, large particles having high wet-proofing and no catalyst, containing from about 30 to 70 volume percent PTFE. The other zone, next to the electrolyte, consists of high surface area, small, catalyzed particles operating in a wetted condition. A single mesh current collector is attached to the zone next to the air supply. Such a configuration is still not completely effective, however, and a need remains for a maximum output, minimum flooding electrode for use in metal/air batteries and fuel cells.
In another art area relating to graphite anodes used in the electrolysis of aqueous alkalide metal halide electrolyte, Currey et al., in U.S. Pat. No. 3,580,824, shows vacuum-impregnating a graphite anode with ferric or ferrous chloride, nitrate, acetate, or formate, and then calcining the impregnated graphite for up to four hours at 800.degree. C. to 2000.degree. C., in an inert atmosphere, to produce varying amounts of alpha iron and alpha-Fe.sub.2 O.sub.3, in-situ in the graphite pores. The iron is used partly as a substitute for oil impregnate, to reduce the consumption of the anode during cell operation, the iron apparently preventing wetting of the interior of the anode by the liquid electrolyte. Such a process would make the electrode substantially electrolyte-impermeable.
Liu, et al., in U.S. Pat. No. 4,341,848, attempted to reduce weight and cost by using elemental iron to replace some or all of the oxygen evolution material. In addition, that gas diffusion electrode maintains a proper balance of electrolyte permeability because the elemental iron provides an electrolyte storage surface. Also, that bifunctional gas diffusion electrode possesses hydrophilic layers, including integrally-contained current collectors, which are press-bonded to hydrophobic layers. The hydrophobic layers are impervious to the electrolyte but capable of permitting gas and oxygen diffusion. However, although the oxygen evolution catalysts in that electrode do not change composition with time, the elemental iron has limited stability during long-term bifunctional cycling. This change in composition limits the life of that electrode due to electrolyte leakage.
Liu, et al., in U.S. Pat. No. 4,444,852, attempted to reduce electrolyte flooding and increase battery output by utilizing at least two bonded, catalytically active material sections in the electrode active layer, each comprising active material preferably bonded to, and at least partially impregnating, a supporting porous metal current collector. Furthermore, each active metal section is equally electrochemically active, and contains catalyst and a blend of hydrophobic agglomerates and hydrophilic agglomerates. However, the two bonded layers do not include a fissured, mud-caked or layer which has been baked to remove nearly all of the surfactant, thereby producing a barrier which is at least partially hydrophobic, and which prevents early electrolyte flooding and electrode failure.